Precision resistor



Aug 3 1965 H. A. CRAMER ETAL 3,199,058

PRECISION RESISTOR Filed March 9. 1962 RN w.. l SE n w Qm` .GWJ WML A l"1 \\\\C\Q Mmmm V .T MAE D M V M A W U 0 L .www .d N u W N NN m7 X s 2,.m vm x hm @n m j I m Nm, mm., ov CQ \\\\\N\\\ CX Ov y s mi N 3V www mUnited States Patent C) M 3,199,053 PRECISIN RESHSTR Howard A. Cramerand William E. McLean, Independence, Kans., assignors to ElectraManufactdring Company, Kansas City, Mo., a corporation of MissourilFiled Mar. 9, 1962, Ser. No. 178,631 S Claims. (Cl. 33m- 237) Thepresent invention relates to t-he encapsulation of electrical componentsand more particularly to precision resistors of the hermetically-sealedtype.

It is .an object of the present invention to provide an encapsulatedresistor in which the resistance element is hermetically sealed in arigid nonconducting cylinder but which is capable of withstanding wideand abrupt changes in temperature without failure or without the settingup of linternal strains producing a condition of incipient failure. Itis a general object of the .invention to provide an encapsulatedprecision resistor having reliability of the lhighest order, approachingabsolute, and which is suitable for use wherever continued operation inthe face of sudden environmental changes is essential fas, for example,in guided missiles, satellites, or the like. It is an object, therefore,to produce an encapsulated resistor which is capable of enduring extremephysical or thermal shock and which is capable of withstandingelectrical overload.

It is one of the specific objects of the invention to produce anencapsulated resistor which is immune to relative dimensional changesoccurring in the resistance element and in the cylinder which enclosesit.

Other features and advantages of the invention will become apparent uponreading the attached detailed description and upon reference to thedrawings in which:

FIGURE 1 shows a resistor of the wire pigt-ail type to which the presentinvention has `been applied.

FIG. 2 is an enlarged axial section of the resistor shown in FIG. 1.

FIG. 3 shows the setup employed in making the terminal assembly employedin the device of FIG. 2.

FIG. 4 is an enlarged axial section of a resistor embodying a modiedconstruction.

While the invention has been described in connection with certainpreferred embodiments, it will be understood that we do not intend tolimit the invention to the embodiments disclosed but on the contrary weintend to cover the various alternative and equivalent constructionsincluded within the spirit and scope of the appended claims.

Turning now to the drawings a resistor constructed in accordance withthe present invention has a body 1t) with axially extending lead wires11, 12, the form factor being similar to resistors of conventional lilmor composition type. The details of construction are revealed in theenlarged axial section, FIG. 2. Here it will be noted that the resistorincludes a circuit element, here shown as a resistor element 15, whichis cylindrical in shape having a resistive film 16 which may be severedby a shallow helical groove 17. For the purpose of making contact withthe film 16, resistor terminals in the form of metal end caps 21, 22 areprovided which are snugly fitted to the ends of the resistor elements.

For the purpose of encapsulating and sealing the resistor element 15, animpervious cylinder 25 is used formed of rigid non-conducting material,the cylinder having a diameter which is sufficiently larger than theresistor element to dene an annular inner space 26.

In accordance with the present invention thin-walled flexible diaphragmmembers are provided for sealing the ends of the cylinder 25 and whichare secured to the resistor terminals at a localized area along the axisso that such members are free to yield, with diaphragm action, inresponse to relative expansion between the resistor ele- Patented Aug.3, 1965 ment and the cylinder which surrounds it. In the presentembodiment the diaphragm elements are indicated at 3l, 32 respectively,with the elements being welded or otherwise secured to the resistorterminals 21, 22 at the center as indicated at 33, 34 respectively. Toinsure freedom of movement of the diaphragm elements in response toendwise forces in both directions, the diaphragm elements are spacedfrom the terminals which they engage. In the present instance suchspacing is accomplished by inwardly dimpling or embossing the diaphragmelements -as at .35, 36. Thus it is not possible for the resistorterminals to bottom, face to face, against the diaphragm elements. Ifdesired, and without departing from the invent-ion, the embossment couldbe provided on the resistor terminals 21, 22. :In either event, theembossment not only insures freedom of movement but also serves tolocalize the weld or connection between the adjacent members so that itoccupies a limited area centered on t-he resistor axis. The lead wires11, 1-2 are preferably buttwelded -in axial position against the outsidesurface of the diaphragm members 31, 32 as indicated at v37, 38respectively, -but they may be attached by other suitable means.

For the puprose of providing a good seal and strong mechanicalconnection between the diaphragm members 31, 32 and the ends of thecylinder 25, the diaphragm -members are preferably provided wit-h wallsor flanges 31a, 32a lrespectively having a 4diameter which is justslightly smaller than the inner diameter of the cylinder. The walls aresealed to lthe cylinder by a suitable sealant 40. The seal between thediaphragm members and the tubing may be brought about using any one of anumber of well known glass or ceram-ic to metal sealing techniques. Forexample, the inner end surfaces of the cylinder may be `metalized andthe sealant 40 may consist of a suitable solder. Or, if desired, thesealant may be in the form of frit lcapable of .wetting both thecylinder and the surface of the metal. Finally, when the cylinder is offusible material sealing may be accomplished by direct fusion; in thiscase it is desirable to employ a metal for the diaphragm elements 31,32, which has substantially the same coeiiicient of expansion as thematerial of which the cylinder 25 is made.

In the preferred embodiment of the invention illustrated in FIG. 2, theresistor terminal element 21 and the associated diaphragm element 31 arepreferably in the form of nested cups or wafers having parallel wallsand with a small .cle-arance space being maintained between them so thatthere is minimum interference with relative axial movement.

The terminal assembly at each end of the resistor, for example, theterminal assembly which consists of the terminal 21, diaphragm 31, andlead wire 1.1, may be welded together 'using a simple and effectivewelding setup as shown in FIG. 3. This ligure shows the parts inexploded relation prior to making the weld. Arranged within the terminalelement 21 is a centering jig 41a and a coaxial first electrode 41. Asecond electrode 42, which is shown diagrammatically and which may be ofany desired coniiguration, clamps the lead wire 11. Guide means 43 maybe used for centering the diaphragm element 31 to insure thatconcentricity is maintained. In operation the electrode 42 and the leadwire 1v1 which it engages are brought down into bottomed contact againstthe diaphragm member 31, thus conditioning the electrical circuit andpermitting formation of the welds previously indicated at 33-37.

After the individual electrode assemblies are welded together, they aresecured to the opposite ends of a resistor blank or substrate having aconductive iilm deposited `on the surface. The resistance of the blankmay be brought to a precise value by grooving in a so-called spiralinglathe or by other suitable means. After the resistance of the blank hasbeen adjusted and the electrode assemblies secured thereto, theprotective cylinder is slipped over the assembly and sealed, asheretofore stated, to the diaphragm elements 31, 32.

' While 4the invention has been described in connection with cup-shapedinner terminals 21, 22 at the ends of the resistor element, it will beapparent to one skilled in the art that other terminal congurations maybe ernployed without departing from the invention. For example, in FIG.4 there is shown an alternate construction in which the resistorelectrodes indicated at 2lb, 22h are disc shaped having a centralprojecting portion 21a, 22a projecting into an opening 15e which extendslongitudinally through the resistor element 15a. The elements may bejoined together using techniques similar to those disclosed in FIG. 3.Both structures posess substantially the same features and advantages.

It is found that resistors constructed in accordance with the presentteachings are capable of resisting thermal shocks which would fractureor destroy encapsulated resistors of more conventional design. Forexample, the units disclosed herein have been subjected to suddenchanges in temperature between 65 and +150 C., a difference of 215 C.,without damage. Conventional encapsulated resistors were found to suffera 70% failure rate when subjected to this treatment. The high degree ofreliability of the present design is considered to result from the factthat any applied temperature change tends to act first upon the cylinder25 effectively changing its length, with the differential expansionbetween the cylinder and the resistor element being accommodated by themovement which takes place in the end walls of the diaphragm elements.lf desired, the end walls of the diaphragm elements may beconcentrically waved or convoluted to provide additional flexibility.However, this has not been found to be necessary in practice.

lt has been found that resistors of the present design are also capableof resisting severe shock and vibration without failure. Some of theseverity of the shock is, of course, taken up by the lead wires 11, 12.but it is believed that substantial added resistance to shock isprovided by oating mount provided by the diaphragms.

From an electrical standpoint it is found that the present resistors arecapable of handling over-current conditions in excess of those which canbe withstood by ordinary precision resistors. The good thermal couplingbetween the resistor terminals and diaphragm elements assists inconducting the heat outwardly where it can be effectively radiated andconducted into the environment. Moreover, any differential expansionbetween the resistor element and the cylinder which encloses it broughtabout by the conducted current is immediately taken up by diaphragmaction.

Since the reliability of the present units approaches absolute, suchunits are ideally suited for use in computers, guided missiles orsatellites, wherever the highest order of reliability is essential.

Notwithstanding the above features and advantages it is noteworthy thatresistors of the present design may be manufactured at low cost. lt willbe appreciated by one skilled in the art, and having in mind thediscussion of FIG. 3 above, that welding, assembly, and sealing may allbe easily automated enabling the present resistors to sell for littlemore than resistors of conventional type.

While the enclosing cylinder 25 may be made of glass, it will beapparent that the invention is not limited thereto and that the membermay be made of ceramic material or any other rigid non-porous materialwhich permits an effective seal with respect to the diaphragm members.

We claim as our invention:

1. An encapsulated resistor or the like compr-ising, in combination, acircuit element, cooperating inner and outer terminal caps at each end`of the element, a rigid cylinder surrounding said element, said innerterminal caps being connected to the element and providing electricalconnection therewith and said outer terminal caps being dimensioned toengage the ends of the cylinder for sealing thereto, the outer terminalcaps being in the form of thin exible metal diaphragms having aninwardly projecting central protuberance secured to the cooperatinginner caps over a limited area at the axis to Itake up differentialexpansion of the circuit element and the cylinder which surrounds itincident to abrupt temerature changes, and axial lead wires secured tothe outer caps.

2. An encapsulated resistor comprising, in combination, a cylindricalresistance element, inner .and outer nested terminal caps at each end ofthe element, a rigid cylinder surrounding said element, said innerterminal caps being dimensioned to engage the element and to provideelectrical connection therewith and said outer terminal caps beingdimensioned to t snugly with respect to the ends of the cylinder forsealing thereto, the outer terminal caps being made of thin flex-iblemetal and having an embossed central portion secured to the innerterminal caps over a limited area at the axis so that relative movementbetween the adjacent caps may occur incident to differential expansionbetween the element and the cylinder incident to abrupt temperaturechanges, lead wires engaging the centers of the outer caps, the leadwire, inner cap and outer cap at each end being coaxially fusedtogether.

3. An encapsulated resistor or the like comprising, in combination, acylindrical circuit element, inner and outer nested terminal caps ateach end of the element, a rigid cylinder surrounding said element,sa-id inner terminal caps being dimensioned to engage the element and toprovide electrical connection therewith and said outer terminal capsbeing dimensioned to lit snugly w-ithin the ends of the cylinder forsealing thereto, the outer terminal caps being made of thin flexiblemetal and having a concave central portion secured to the inner terminalcaps over a limited area at the axis so that relative movement betweenthe adjacent caps may occur incident to dilerential expansion betweenthe element and the cylinder incident to abrupt temperature changes, andmeans for making external electrical connections to the center of theouter terminal caps.

4. An encapsulated resistor or the like comprising, in combination, anelongated circuit element having conductive end terminals axiallypositioned thereon, a rigid protective cylinder surrounding saidelement, a cup-shaped diaphragm member at each end of the circuitelement, each of said diaphragm members having a generally radiallydisposed imperforate metallic central portion and a peripherallydisposed axially directed portion, said axially directed portion of eachdiaphragm member being secured to respective opposite ends of saidcylinder and sealingly engaged therewith so as to define a totallyenclosed chamber surrounding said element, said imperforate centralportion of each of said diaphragm members being in the form of adiaphragm sufficiently thin as to deform axially in both directions whenforce is applied to the center thereof, each diaphragm member beingsecured to the adjacent terminal over a limited area at the center andallowing for differential expansion between the circuit element and thecylinder, and external electrical Contact means secured to saiddiaphragm members with freedom for movement therewith as said diaphragmmembers ex due to differential expansion between said circuit elementand said cylinder.

5. An encapsulated resistor or the like comprising, in combination, acylindrical resistor element having conductive end terminals axiallypositioned thereon, a hollow cylinder telescoped thereover, a diaphragmmember at each end of the resistor, each of said diaphragm membershaving a generally radially disposed imperforate metallic centralportion and a peripherally disposed axially directed portion, saidaxially directed portion of each diaphragm member being secured torespective opposite ends of said cylinder and sealingly engagedtherewith so as to define a totally enclosed chamber surrounding saidelement, said imperforate central portion of each of said diaphragmmembers being in the form of a diaphragm suiciently thin as to deformaxially in both directions when force is applied to the center thereof,each diaphragm member being secured to the adjacent resistor terminalover a limited area at the center and allowing for differentialexpansion between the resistor element and the cylinder, and externalelectrical contact means secured to said diaphragm members with freedomfor movement therewith as said diaphragm members flex due to dicrentialexpansion between said circuit element and said cylinder.

6. An encapsulated resistor or the like comprising, in combination, acylindrical circuit element, cooperating inner and outer terminal capsat each end of the element, a rigid cylinder surrounding said element,said inner terminal caps being dimensioned to engage the element and toprovide electrical connection therewith and said outer terminal capsbeing dimensioned to engage the ends of the cylinder for sealingthereto, at least one of the caps at each end having an axial projectionproviding a limited welding area for the cooperating cap and the outerterminal caps being in the form of diaphragms of thin lexible metal sothat relative movement may occur between the cooperating caps incidentto differential expansion between the circuit element and the cylinderupon abrupt change in temperature, and axial lead wires welded to thecaps.

7. An encapsulated resistor or the like comprising, in combination, acylindrical circuit element, cooperating inner and outer terminal capsat each end of the element, a rigid cylinder surrounding said element,said inner terminal caps engaging the element to provide electricalconnection therewith and said outer terminal caps being dimensioned totelescope within the cylinder for sealing to the ends thereof, the outerterminal caps being made of thin flexible metal and secured to the innerterminal caps at the axis and at least one of the cooperating caps ateach end being dimpled to provide a limited welding area with the capsspaced apart a small distance axially so that relative movement mayoccur between them incident to differential expansion between thecircuit element and the cylinder upon abrupt changes in temperature, andaxial lead wires welded to the caps.

S. An encapsulated resistor or the like comprising, in combination, acylindrical circuit element, inner and outer nested terminal capsmounted on one end of said element, a rigid protective cylinder ofimpervious material surrounding said element and spaced therefrom, saidinner terminal cap being dimensioned to engage said element so as toprovide electrical connection therewith, said outer terminal cap beingdimensioned to t snugly within one end of the cylinder for sealingengagement therewith, means for sealing the other end of said cylinderand for providing electrical engagement with the other end of saidcircuit element, said outer cap being made of thin flexible metal andhaving an embossed central portion secured to said inner cap over alimited area in the region of the axis thereof so that relative movementbetween the adjacent caps may occur incident to differential expansionbetween said element and said cylinder resulting from abrupt temperaturechanges.

References Cited by the Examiner UNITED STATES PATENTS 2,046,922 7/ 36Minnium 338-273 2,215,587 9/40 Kerschbaum 338-274 X 2,402,773 6/ 46Ragatz 338-274 X 2,407,171 9/ 46 McFarren 338-274 X 2,416,599 2/47Victoreen 338-273 X 2,553,875 5/51 Shaw 338-274 X 3,064,070 11/62Douglass et al 338-274 X FOREIGN PATENTS 207,597 2/ 40 Switzerland.580,626 10/52 Great Britain.

RCHARD M. WOOD, Primary Examiner.

1. AN ENCAPSULATED RESISTOR OR THE LIKE COMPRISING, IN COMBINATION, ACIRCUIT ELEMENT, COOPERATING INNER AND OUTER TERMINAL CAPS AT EACH ENDOF THE ELEMENT, A RIGID CYLINDER SURROUNDING SAID ELEMENT, SAID INNERTERMINAL CAPS BEING CONNECTED TO THE ELEMENT AND PROVIDING ELECTRICALCONNECTION THEREWITH AND SAID OUTER TERMINAL CAPS BEING DIMENSIONED TOENGAGE THE ENDS OF THE CYLINDER FOR SEALING THERETO, THE OUTER TERMINALCAPS BEING IN THE FORM OF THIN FLEXIBLE METAL DIAPHRAGMS HAVING ANINWARDLY PROJECTING CENTRAL PROTUBERANCE SECURED TO THE COOPERATINGINNER CAPS OVER A LIMITED AREA AT THE AXIS TO TAKE UP DIFFERENTIALEXPANSION OF THE CIRCUIT ELEMENT AND THE CYLINDER WHICH SURROUNDS ITINCIDENT TO ABRUPT TEMPERATURE CHANGES, AND AXIAL LEAD WIRES SECURED TOTHE OUTER CAPS.